Scientists collected 16 years of data from RXTE, a decommissioned low-earth orbit satellite that was equipped with instruments that measured variations in X-ray sources. This treasure trove of data revealed a dozen instances when the X-ray signal dimmed for periods of time ranging from hours to years, presumably when a cloud of dense gas passed between the source and satellite.

"One of the great unanswered questions about AGN is how gas thousands of light-years away funnels into the hot accretion disk that feeds the supermassive black hole," Alex Markowitz, an astrophysicist at the University of California, San Diego and the Karl Remeis Observatory in Bamberg, Germany, said in a statement. "Understanding the size, shape and number of clouds far from the black hole will give us a better idea of how this transport mechanism operates."

This is the first study to survey the environments around supermassive black holes and is the longest-running AGN-monitoring study yet performed in X-rays. The team, publishing a paper in the journal Monthly Notices of the Royal Astronomical Society, described the various properties of the clouds, including how they ranged in size.

The clouds found orbit a few light-weeks to a few light-years from the center of the active galactic nuclei. The team said a cloud found in a spiral galaxy in the direction of the constellation Centaurus appears to be in the midst of being torn apart by tidal forces.

"In 2008, the AGN dimmed twice over a period of 11 days and did not reach its typical X-ray brightness within that period," co-author Mirko Krumpe, of the European Southern Observatory in Garching, Germany, said in a statement. "This could be caused by an elongated, filamentary cloud, perhaps one that is in the process of being torn apart by the black hole."

This study has helped triple the number of cloud events previously identified by the RTXE satellite. Instruments on board the satellite measure X-ray emissions on timescales as short as microseconds to as long as years. The satellite was decommissioned by NASA back in 2012, but it still had 16 years of successful operation.

“Because RXTE performed sustained observations of many of these AGN, our research is sensitive to a wide range of cloud events, from those as brief as five hours to as long as 16 years," co-author Robert Nikutta, a theorist at Andrés Bello University in Santiago, Chile, said in a statement.